The invention relates generally to light-emitting devices, and, more particularly, to an electrically-pumped, vertical-cavity surface-emitting laser (VCSEL) having multiple active regions.
Light-emitting devices are used in many applications including optical communication systems. Optical communication systems have been in existence for some time and continue to increase in use due to the large amount of bandwidth available for transporting signals. Optical communication systems provide high bandwidth and superior speed and are suitable for efficiently communicating large amounts of voice and data over long distances. Optical communication systems that operate at relatively long wavelengths on the order of 1.2 micrometers (xcexcm) to 1.6 xcexcm are generally preferred because optical fibers generally have their lowest attenuation in this wavelength range. These long wavelength optical communication systems include a light source capable of emitting light at a relatively long wavelength. Such a light source is a vertical-cavity surface-emitting laser (VCSEL), although other types of light sources are also available.
VCSELs are generally characterized by a pair of mirrors, generally referred to as distributed Bragg reflectors (DBRs), between which an optical cavity is located. The entire structure can be formed over a substrate wafer by a process called organometallic chemical vapor deposition (OMVPE), sometimes referred to as metal organic chemical vapor deposition (MOCVD). The optical cavity generally also includes spacer layers and an active region. The active region typically includes one or more quantum wells. The quantum wells, which typically include a quantum well layer sandwiched by a pair of adjacent barrier layers, are the layers into which carriers, i.e., electrons and holes, are injected. The electrons and holes recombine in the active region and emit light at a wavelength determined by the material layers in the quantum well. The quantum well layer typically comprises a low bandgap semiconductor material, while the barrier layers typically have a bandgap higher than the bandgap of the quantum well layers. In this manner, when the device is subject to forward bias, electrons and holes are injected into and trapped in the quantum well layer and recombine to emit coherent light at a particular wavelength.
The spacer layers separate the active region from the DBRs. The DBRs are formed using alternating layers of materials that have different refractive indices. The reflectivity of the DBRs is determined by the refractive index of the materials of the layers constituting the layer pairs, with one DBR being slightly less reflective than the other. The light generated in the active region is emitted from the VCSEL through the less reflective DBR.
VCSELs having multiple active regions have also been produced. The number of active regions in a VCSEL determines the threshold gain of the VCSEL. Before light is emitted from a VCSEL, many losses inherent in the VCSEL have to be overcome. Losses in a VCSEL are created by the mirrors, diffraction, and distributed losses. Losses in the mirrors are due to the mirror reflectivity being less than 100%. Indeed, if the mirrors were 100% reflective, light could not be emitted form the VCSEL. Diffraction losses are caused when the emitted light expands as it propagates away from a guiding aperture in the VCSEL. Distributed losses are caused by scattering and by absorption of light in the VCSEL structure.
To overcome some of these losses, optically-pumped VCSELs may include multiple active regions. Early optically-pumped VCSELs had three active regions, each having five quantum wells. The multiple quantum wells were used to overcome the scattering loss in the mirrors because the mirrors were constructed of dielectric layers with slightly rough surfaces. Generally, if the overall loss in the VCSEL can be minimized, fewer quantum wells are needed. Furthermore, high-temperature performance of the VCSEL can be improved (i.e., T zero can be increased) by using multiple quantum wells.
However, in a VCSEL there is a limit to the number of quantum wells that can fit under one maximum of the standing wave that exists in the optical cavity. If too many quantum wells are added, the outer quantum wells do not coincide with the optical mode of the standing wave, and the quantum wells do not fully contribute to the modal gain of the VCSEL. To overcome this, the multiple quantum wells can be arranged in what is referred to as a resonant periodic gain (RPG) structure. An RPG structure includes two or more active regions, with the active regions lying under adjacent maxima in the standing wave. This RPG structure is used to maximize the optical gain of VCSELs in which the threshold gain (i.e., the gain to overcome all losses) of the VCSEL is relatively high.
In a particular implementation of a VCSEL, one of the DBRs is movably arranged so that the output wavelength of the VCSEL can be adjusted. Such a VCSEL is referred to as a tunable VCSEL. As known in the art, one of the mirrors is located over a cavity spacer layer and can be moved by an electrostatic motor, thereby allowing the output wavelength of the VCSEL to be changed.
Light is generated in the just-described tunable VCSEL by optical pumping. Optically-pumped long-wavelength tunable VCSELs generally have undoped material layers and use a high-power laser diode having an output of approximately 980 nanometers (nm) to generate carriers in the VCSEL""s active regions. The carriers recombine in the active regions and the VCSEL emits light at the desired wavelength.
One of the drawbacks of an optically-pumped VCSEL is the high complexity involved in aligning the output of the pumping laser diode with the VCSEL. Another drawback in some applications is that all of the active regions in the VCSEL are stimulated using the same light. Finally, the overall efficiency of an optically-pumped laser is lower than that of an electrically-pumped laser because an optically-pumped laser is generally larger and more costly than an electrically-pumped laser. An optically-pumped laser is larger because there are two devices, and more costly because the two devices must be carefully aligned with respect to one other.
Electrically-pumped VCSELs include a p-i-n junction structure in which the intrinsic material that forms an active region is sandwiched between layers of p-type and n-type material. For optimal light emission, the active region should be located at a peak of the standing wave (the axial or longitudinal mode) generated in the optical cavity. However, in a VCSEL having multiple active regions, each of the active regions is separated by several hundred nanometers. In a VCSEL that includes a single p-i-n junction, there is no conventional solution for electrically injecting carriers uniformly from the single p-i-n junction into the multiple active regions.
As mentioned above, multiple active regions are used in an optically-pumped VCSEL to overcome losses and lower the laser threshold current, i.e., the minimum current through the pumping laser at which light is generated by the VCSEL. The multiple quantum wells are placed under adjacent maxima in the standing wave. These maxima lie exactly xc2xd-wavelength apart within the cavity assuming a cavity thickness greater than one-wavelength. For example, assume a cavity thickness of 2, 3 or n-xcex, where xcex is the wavelength of the emitted light in the material of the cavity. At a wavelength of 1.5 xcexcm and a refractive index of 3.5, the two adjacent active regions are separated by  greater than 200 nm. Assuming there are two active regions, and the p-i-n junction is located around one of them, the other active region lies  greater than 200 nm from the p-i-n junction. The number of electrons and holes injected into the distant active region will be lower than the number injected into the active region closer to the p-i-n junction. Generally, a single p-i-n junction cannot uniformly excite two separate active regions. The homogeneity of the injected carrier density is better if the active regions are closer together. However, in a VCSEL, the internal intensity profile of the longitudinal optical mode dictates the spacing of the active regions. Therefore, a VCSEL structure prevents uniform pumping of multiple active regions with a single junction.
The parameters characterizing the non-homogeneity of electrically pumping multiple active regions with a single p-i-n junction are the xc2xd-wavelength spacing distance between the active regions, which is determined by the wavelength, the refractive index, and the minority-carrier diffusion lengths. The minority-carrier diffusion length characterizes the distance from the p-i-n junction over which the injected carrier density decays. If the xc2xd-wavelength spacing distance is more than a small fraction of the minority carrier diffusion length, the pumping of multiple active regions with a single p-i-n junction will be non-homogeneous.
Therefore, an effective electrically-pumped alternative to an optically-pumped, VCSEL having multiple active regions is desirable.
The invention provides several embodiments of an electrically-pumped VCSEL having multiple active regions. The invention allows the active regions to be pumped electrically and to be located at the maxima of the standing wave in the optical cavity.
In one embodiment, the invention is an electrically-pumped vertical-cavity surface-emitting laser (VCSEL), comprising a pair of distributed Bragg reflectors (DBRs), an optical cavity located between the DBRs, active regions located in the optical cavity, and p-i-n junction structures equal in number to the active regions. Each of the active regions constitutes the intrinsic (i) layer of a corresponding one of the p-i-n junction structures.
Embodiments of the invention also provide an electrically-pumped, tunable VCSEL having a number of different p-i-n junction and electrode arrangements, which, in various embodiments, allow independent biasing of the multiple active regions, and, in other embodiments, allow simplified electrical connections.
Other features and advantages in addition to or in lieu of the foregoing are provided by certain embodiments of the invention, as is apparent from the description below with reference to the following drawings.